What over-the-counter sedatives or relaxers would you recommend for a dog?

We have an adopted 7-8 year old St. Bernard that becomes frantic when he hears thunder. Before we knew this about him, we had left him at home to go grocery shopping and came back to massive holes in our walls with his mouth and paws bleeding profusely. We live in Arizona and our monsoon season is coming in by next week and these storms produce some ferocious thunder. We spoke to his vet and he told us to try over-th-counter and see how that goes before we try prescriptions or shots. I forgot to ask him what brand he preferred before I left. Thanks for your help!